Dallas police have received an anonymous threat against officers across the city, and the department was taking extra safety precautions, it said on Saturday.

"The Dallas Police Department received an anonymous threat against law enforcement across the city and has taken precautionary measures" to heighten security, Dallas police said in an emailed statement.

Police headquarters have been placed on lockdown and are surrounded by SWAT team vehicles, according to local media, which cited unnamed sources.

The department said it would not immediately confirm or deny whether its headquarters was under lockdown.